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Topic: Ring opening of aziridine with PhMgX (Read 3018 times)

Rhodium

If ethylene oxide is treated with phenylmagnesium bromide and hydrolyzed, 2-phenethylalcohol will result.

What if one treated N-tosyl-aziridine* with phenylmagnesium bromide? Would 2-phenethylamine result directly after the hydrolysis? If so, then we have a GREAT novel route to all our favorite phenethylamines...

*Aziridine is the nitrogen analog of ethylene oxide, and it needs to be protected with tosyl, or the grignard will be quenched.

oxycodfish

Thank goodness for ring strain! Looks like a good possibility, at least based on my unexpert estimation. Yields for the closest analog I could find to this aziridine reaction are around 60%, couldn't find any actual *quantitative* data on reaction rates though. Then again, I'm currently library-less and journal-less, so my search capabilities aren't what they used to be. Hopefully this situation will be rectified soon. I'm rambling -- in any case, I'll be interested to hear any results (theoretical or empirical) in this line of research.

jim

A classic way to get around the quenching of the Griganrd reagent by the desired end product is reverse addition.

That is, slowly add (AKA drip) the Grignard reagent to the vat of the compound to be acted upon. The Grignard reacts with the most availible substance, which is usually the starting compound, and not wit the end product. In this case that might be a little hard however, because I believe that Aziridine is a gas at room temperature, but I could be wrong. Even if the Aziridine is a gas one could dissolve the Aziridine in an inert liquid.

AS A CRAZY SIDE NOTE: I recently discovered that poly(ethylene glycols) can be very easily ETHERIFED, that is @ room temperture, with KOH (and maybe NaOH), and an alkyl halide, the poly(ethylene glycol)'s OH groups can be can alkylated creating ether. Perhaps the etherifed poly(ethylene glycols) can be used as solvents for Grignard reactions since they are close analogs to ethyl ether, are known to coordinate well with metal ions, and are at the very least totally inert to Grignard compounds.

The above is an article about exactly this reaction. They used the diethylphosphite protected 1-methylaziridine, and got the ring-opened product in 87% yield, and after hydrolysis they isolated about 30% amphetamine (on a 0.03 mole scale, it should be better on a larger scale).

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Greensnake

Yeah, this route is fairly doable. I faintly recall that copper modified lithiumorganics were used and probably this must work with Grignards too. Ok, it's good idea for research purposes, but GOD FORBID anybee tries to mess with aziridines at home. Aziridine is toxic, carcinogenic and what else, and in some aspects make potassium cyanide look like innocent food additive.

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SarahTonin

I like the idea of this in principle. However, in practice I have found it difficult to generate aromatic Grignard reagents with electron-donating groups. Likely problem is destabilization of the pseudocarbanion as a result of the electron donation. Same thing happens in trying to make aryllithiums. This could be a drawback considering most of the desirable compounds contain functionality such as MeO, Me, etc. If you can get it to work, awesome, but I have an alternate suggestion along the same lines. Here goes:

It is well documented that styrene and substituted styrenes will undergo catalytic nitrene transfer aziridination reactions. The nature of the substitution seems to have little or no effect on the yields. Any number of Cu-based catalysts are highly effective, with many giving QUANTITATIVE yields of the aziridine product. The nitrene precrsor is extremely easy to make, providing you can get a hold of some Iodobenzene diacetate. The precursor is synthesized as follows:

As for the aziridination reactions, the only drawback is a potentially difficult synthesis of your desired styrene precursor. However, these things can be done. Regardless, these reactions do work and they work well. Basically they go like this:

Reaction works usually by stirring overnight at room temp in CH3CN. Workup involves chromatography, but this forum is for the pro bees anyway. Once you have this aziridine, simple hydrogenolysis/ring opening should yield the appropriate PEA.

Sorry I can't give refs right now, but I will provide in the next few days. Got em written down but forgot to bring em.

AbstractThe regioselectivity ratio RS = normal:abnormal opening of activated 2-methylaziridines 2 by nucleophiles is found to range from 0.10 to unmeasurable large (only normal opening = substitution at CH2 by strongly basic carbanions). RS is assumed to result from SN2 variants differing in the degree to which bond breaking is ahead of bond making including perhaps synchronous SN2. Bond breaking will be more ahead for the N-CMe bond. High nucleophilic power pushes bond making toward a synchronous process resulting in great RS. The decrease in RS with acyl activation relative to sulfonyl activation is in accord with a flattening of the nitrogen pyramid (planarization effect). The planarization effect is retained in acidic medium by O-protonation: Rs 0.10-0.14 for methanolysis as compared to RS 0.43 for N-protonated sulfonylaziridine 2h. AMI calculations support the planarization hypothesis. - No indication for SET with trityl anion was found.

Reaction of N-Tosyl-2-Methylaziridine with phenylmagnesium bromide

A solution of 2.11 g (10 mmol) of N-Tosyl-2-Methylaziridine (2h) in 100 ml of THF was added dropwise (30 min) to a boiling solution prepared from 1.20g (50 mmol) of magnesium turnings and 1.57g (10 mmol) of bromobenzene in 100 ml of THF. Heating at reflux was continued for 5 h. After cooling to room temp. the mixture was poured onto ice. Five extractions with 50 ml of dichloromethane each yielded 2.46 g (82%) of N-(1-Methyl-2-phenylethyl)-4-toluenesulfonamide (29h) as an oil.

Rhodium

(S)-N,O-Ditosylalaninol 6cTosyl chloride (9.53 g, 40 mmol) was added in portions over 40 min to (S)-alaninol (5a, 1.50 g, 20 mmol) in pyridine (50 mL) at 0°C. Stirring was continued for another 3 h, then the flask was placed at -30°C overnight. Ice (200 g) was added followed by concd HCl (60 mL). The water/pyridine solution was extracted twice with EtOAc. The combined organic phases were washed with saturated aqueous NaHCO3, water and brine, and finally dried (MgSO4). The solvent was evaporated, leaving a sticky yellow substance. Recrystallisation from hot ethanol yielded 4.77 g (62%) of 6c as a yellow powder.

(S)-N-Tosyl-2-methylaziridine 7cSulfonamide 6c (4.77 g, 12.4 mmol) in dry THF (40 mL) was added dropwise to a suspension of washed (hexane) sodium hydride (18.6 mmol) in dry THF (10 mL). The resulting mixture was stirred for 2 h at room temperature, with a lot of foaming. The reaction was quenched by adding water (50 mL) and brine (50 mL). The aqueous phase was extracted 3 times with diethyl ether, the combined ether phases were washed with 2 M NaOH and dried (MgSO4), and the solvent evaporated, leaving a yellow liquid. The liquid was filtered through silica (6 cm column, eluent: hexane:EtOAc 80:20). Evaporation left 1.24 g (47%) of 7c as a white solid.

Synthesis of N-Tosyl-2-methylaziridineOrg. Lett. 4(6), 949-952 (2002)

(S)-N-Tosyl-2-methylaziridine 7cTo a solution of L-alaninol (8 mmol) and triethylamine (3.5 equiv) in dry acetonitrile (80 mL) was slowly added p-TsCl (2 equiv) and DMAP (0.1 equiv) at 0°C with stirring for 30 min. The reaction mixture was stirred at room temperature for 4-5 h. The solvent was removed under reduced pressure and dissolved with EtOAc (100 mL). The organic layer was washed with brine (2x40 mL). The organic layer was dried over anhydrous MgSO4 and filtered, and the filtrate was concentrated under reduced pressure. The crude product was purified on a silica gel column chromatography to give white solid 13 (1.62 g, 82% yield).

(+)-(2S)-2-Methyl-1-(p-tolylsulfonyl)aziriridine (17)Helv. Chim. Acta. 83, 2594-2606 (2000)A 50-ml pear-shaped flask was charged with a mixture of TsCl (4.19 g, 22 mmol), pyridine (3.5 ml), and CH2Cl2 (6.5 ml). The mixture was cooled to 0°C, and (-)-(2S)-2aminopropan-1-ol (0.751 g, 10.0 mmol) was added dropwise. The mixture was allowed to warm to RT. and stirred for 16 h. The mixture was then poured into a separatory funnel containing 50 ml of cold 1N HCl and 25 ml of CH2Cl2. After extraction, the org. layer was separated and washed with saturated CuSO4 solution (1x25 ml) and brine (1x25 ml); each aqueous layer was back-extracted with CH2Cl2 (2x25 ml). The organic fractions were combined, dried (MgSO4), and concentrated in vacuo to give 3.9 g of a greenish oil, which was taken up in dry acetone (50 ml) and stirred over 5.0 g of K2CO3 for 14 h. This mixture was filtered through a plug of Celite and concentrated in vacuo to give 3.4g of a yellow oil, which was purified by flash chromatography hexane) to give 1.99 g of a white solid, which was recrystallized from refluxing petroleum ether to give 1.86 g (8.8 mmol, 88%) of 17 Colorless needles, mp 57-58°C.

Deprotection of p-ToluenesulfonateA solution of 0.2 mmol of an N-Tosylamide in 5 ml of dry 1,2-dimethoxyethane (DME) was cooled to -50°C, and the above naphthalide solution was added dropwise with vigorous stirring. Addition (ca. 5 ml) was continued until a persistent green color was achieved. The mixture was allowed to warm to 0° and then quenched by addition of 1 ml of EtOH. The mixture was concentrated at reduced pressure to give a solid white residue, which was purified by flash chromatography to afford the deprotected amine (90%+ yield) as a slightly yellow oil.

AbstractChiral -aryl(heteroaryl)alkylamines have been prepared from N-tosyl alkylaziridines via regiospecific nucleophilic ring opening and subsequent desulfonylation in good to excellent yields. The corresponding aziridines are easily obtained from commercially available (S)--amino acids, so this method is the first effective route to asymmetric -aryl(heteroaryl)alkylamines.

2.1. Synthesis of starting chiral N-tosyl aziridines

We reasoned that chiral aziridines are convenient synthetic precursors to chiral -aryl(heteroaryl)alkylamines as it is well known that monosubstituted aziridines are attacked by nucleophiles exclusively at the less hindered site. Thus, cleavage of chiral aziridines with aryl and heteroaryl organometallic reagents could offer a straightforward route to chiral -aryl(heteroaryl)alkylamines. However, this transformation has not been employed previously.

The presence of a suitable electron-withdrawing protecting group is necessary for effective ring opening. There are several protecting groups that could serve as activators of aziridine cleavage such as diphenylphosphinyl (Dpp), diethoxyphosphoryl and sulfonyl. In our synthesis, we chose the tosyl group as the activator. It is known that N-tosyl aziridines could be easily synthesized from commercially available natural -amino acids using a one-pot modification of Craig's protocol[9] and following this procedure, we prepared N-tosyl aziridines from phenylalanine, leucine and valine. In the case of the alanine derivative, ring closure was unacceptably slow. To overcome this problem the last step of the procedure was modified and ring closure was carried out using potassium hydroxide in methanol.

2.2. Nucleophilic ring opening of N-tosyl aziridinesReadily available (2S)-benzyl-N-tosylaziridine was chosen as a model substrate in the screening experiments searching for the best cleaving organometallic reagent. Various lithium, zinc and magnesium derivatives of thiophene were used as the organometallic reagents. It was found that the desired -aryl(heteroaryl)alkylamine was obtained only by cleavage of the aziridine ring with the Grignard reagent in the presence of catalytic amounts (15 mol%) of copper(I)iodide. The use of alternative organometallic reagents did not furnish the required product.

The reaction occurred smoothly to give N-tosylamides arising from attack of nucleophile at the less substituted ring carbon of the aziridine in good yields (63–89%). A mixture of regioisomers was detected only in the case of reaction with 3-indolyl magnesium bromide, which could be explained by the fact that ether was used as the solvent in this reaction due to the poor solubility of 3-indolyl magnesium bromide in THF. In all of the other experiments THF was used as the solvent.

Experimental

4.2. General procedure for ring opening of N-tosyl aziridines with Grignard reagents

To Mg (0.17 g, 7 mmol) in a flame-dried flask under Ar THF (10 mL) was added. The solution of corresponding arylbromide (7.1 mmol) in THF (5 mL) was added dropwise. The mixture was stirred for 30 min at rt, then cooled to -30°C and dry CuI (0.2 g, 1.05 mmol) was added. The suspension was stirred for 30 min at -30°C, then cooled to -78°C. A solution of the corresponding 2-alkyl-1-[(4-methylphenyl)sulfonyl]aziridine (3.5 mmol) in THF (10 mL) was added dropwise. The suspension was stirred at -78°C for 15 min, then at 0°C for 1 h, after which time it was partitioned between NH4Cl and ether, the aqueous layer extracted with ether (3×15 mL), the organic layers dried (Na2SO4), filtered and the solvent removed in vacuo to leave a brown oil which was purified by chromatography on silica gel with hexane–dichloromethane (1:1) as eluent.

4.5. General procedure for desulfonylation

To a suspension of Mg (0.45 g, 20 mmol) in MeOH (5 mL) was added a solution of the corresponding N-tosylalkyl-2-arylamine (2 mmol) in MeOH (10 mL). The resulting suspension was sonicated for 1 h until consumption of the starting material was complete. The reaction mixture was then diluted with aqueous NH4Cl and extracted with ether (3×5 mL). The organic layer was dried over MgSO4 and evaporated. To resulting oil ethanolic solution HCl (0.5 mL, 2 M) was added. Hydrochloride was precipitated, filtered and washed with ether.

The desulfonylation proceeds with 1.5 eq. of iodotrimethylsilane in acetonitrile, refluxing for 3-4 hr in good yields. The mild reaction conditions employed in this deprotection method allow the selective deprotection of sulfonamides in the presence of N-alkyl and N-benzyl groups. In conclusion, the reagent used for this cleavage is inexpensive, non-toxic and the cleavage is carried out under mild and neutral conditions.

Typical procedure: To a suspension of sodium iodide (1.5 mmol) in acetonitrile (10 mL) chlorotrimethylsilane (1.5 mmol) was added dropwise and stirred for 10 minutes at 0°C under a N2 atmosphere. To this stirred suspension, a solution of N,N-diphenylsulfonamide (1 mmol) in acetonitrile (5 mL) was added and refluxed for 3 hrs. The reaction mixture was quenched with water and extracted with ethyl acetate (25 mL). The organic layer was washed with 10% sodium thiosulphate solution, brine, dried over anhydrous Na2SO4 and concentrated under vacuum to give the crude product. This was purified by column chromatography to afford pure N,N-diphenylamine (88% yield).

SummaryAn extremely convenient desulfonylation method of primary, secondary, tertiary alkyl and vinyl phenyl sulfones was developed by using magnesium in ethanol in the presence of catalytic amount of mercuric chloride to give the corresponding alkanes and alkene in almost quantitative yields.

AbstractAziridines react smoothly with arenes in the presence of a catalytic amount of indium triflate at ambient temperature to afford the corresponding -aryl amine derivatives in excellent yields with high regioselectivity.____ ___ __ _

where (chiral) beta-aminoalcohols are treated with TsCl/NaOH(aq) to form the corresponding N-Ts-Aziridines, which are then ring-opened by the reaction with Phenyllithium, to form the corresponding (chiral) alpha-substituted phenethylamines.

AbstractA novel, convenient synthesis of N-(diethoxyphosphoryl)aziridine is described. Application of this compound as a synthetic equivalent of an a2 type synthon for aminoethylation of Grignard reagents is demonstrated.